What term best describes the Symbiodinium species that associates with coral?

a. Obligate ectosymbiont
b. Obligate extracellular endosymbiont
c. Obligate intracellular endosymbiont
d. Facultative ectosymbiont
e. Facultative extracellular endosymbiont
f. Facultative intracellular endosymbiont